Figure 7: Illustration of WS-H+ WEG
in reservoir of pH 6.9 and pH 12.5, creating a concentration gradient.
b) FT-IR of WS, WS-H+ and WS-H+ on
NaOH reservoir. c) TGA of Natural WS, Treated WS and
WS-H+. d) VOC enhancements of
nano-engineered WS-H+. e) ISCC of
WS-H+ (16 mm X 18 mm) on DI water. f)
ISCC of WS-H+ (16 mm X 18 mm) on
alkaline solution.
The WS-H+ samples were separately immersed into
neutral reservoir of pH 6.9 as well as the alkaline reservoir with a pH
of 12.5. Because of gravity, the top acid solutions travel toward the
reservoir, whereas the solutions in the reservoir move upwards because
of the action of capillary, causing the formation of concentration
gradients as illustrated in Figure 7 (a). Figure 7 (d) confirms the Voc of 960 mV and Figure 7 (e) confirms the maximum Isc of 288 µA when the sample was
submerged in neutral solutions. Figure 7 (d) also confirms the
Voc of 1.21 V when WS-H+ was immersed
in a solution with a pH of 12.5 and the Isc reaches its
maximum value of 1 mA as seen in Figure 7 (f) . The difference
in pH levels between the WS-H+ surface and the
reservoir break apart the surface functional groups unevenly, leading to
the enhanced output voltage and current. On top of that, these strong
acid treatments enhance the surface area and surface
charge.[54,55] When the WS-H+ is
placed on the alkaline reservoir, gravity causes the
H+ ions to travel across the channel and fall into the
reservoir underneath. Consequently, the H+ ions react
spontaneously with OH- ions following the equation of
H+ + OH- = H2O
resulting in depletion of H+ ions. Therefore,
continuous migration of H+ occurs in this acid-base
reaction process. Furthermore, capillary-assisted upward movement of the
OH- ions and gravity-assisted downward migration of
H+ ions would result in the formation of a
concentration gradient. The non-uniform distribution of a strong acid
over the alkaline storage region would lead to an increase in both
output voltage and current, as presented in the Figure 7 (d,
f). A potential difference is also created between the two electrodes
because of the continuous movements of the H+ and
OH- ions. Thus, the WS-H+ devices
achieved a revolutionary open circuit voltage of 1.21 V and a maximum
short circuit current of 1 mA because of the combined influence of
acid-base chemical processes, concentration gradients, streaming, and
evaporation potentials compared with the original (WS-WEG) physically
driven Voc of 620 mV. This substantial contribution from
the chemical reactions and concentration gradients were overlooked in
the previous evaporation-driven WEG
devices.[16,18,56] The porous nutshells can also
be considered as a separation membrane between the acid and base
solutions. To sustain the reactions and concentration gradients the
concentrated acid solutions were introduced on the top surface of the
WS-H+ at a rate of 0.1 ml/h. This remarkably efficient
hydrovoltaic device demonstrates the highest current density of 347.2
μA/cm2, representing a significant advancement.
Underlying operational mechanism of
the
device
The mechanism of this water-induced hydrovoltaic electricity generation
is relatively complex. The basic mechanism involves ionization of the
abundant functional groups on the channel wall of the nutshell surface
upon water adsorption, leading to a negative surface charge. Meanwhile,
the positive ions of the water will be absorbed on the negatively
charged surface of the nutshell (NS), forming an electrical double layer
(EDL),[57] illustrated in Figure 8 (b). The micro/nanochannels of the NS structures are considered as passive
diffusion channels. In response to concentration gradients, passive
diffusion of ions occurs across the shell structure without the need for
outward energy input. The porous structures of the NS facilitate the
movement of ions along their concentration gradient. Directional water
flow inside the channels continues due to the water absorption on one
side and evaporation on the other side. Because of the shearing effect
of the water flow, the positive charged ions start to accumulate on the
evaporation side of the NS. Only the positive ions can effectively
traverse the negatively charged micro/nanochannels due to the strong
electrostatic repulsion exerted by the negatively charged surface,
hindering the penetration of negative ions.[21,56]Due to the separation of charges, potential differences are created
between the electrodes that drive electricity generation through the
phenomenon of streaming potential. The flow direction determines the
positive and negative polarity observed during measurements. An electric
field is established as ions accumulate along the flow of
water.[21,58] The streaming potential increases
linearly with the pressure difference that drives the water flow. The
alteration in polarity inside the device is caused by the transpiration
of water and the transport of ions via the micro/nano channels. As water
drops on the top end, positively charged ions concentrate at the bottom
electrode, generating positive polarity in the flow’s direction. When
water flow is exclusively dependent on the natural evaporation, the flow
reverses from bottom to top, resulting in a polarity inversion that
charges the top electrode positive. The streaming potential is
contingent upon surface chemistry, ion transport, and flow dynamics
within the micro/nano channels. Alterations in flow direction
immediately affect the polarity of the generated voltage.
The phenomenon of streaming potential and streaming current can be
explained by Equation 1 and Equation 2 and illustrated
in Figure 8 (a) . Considering the cross-sectional area A as(πd2)/4 , the vertical pressure differenceΔP as (4γcosθ)/d , the ultimate streaming potential and
current can be expressed by Equation 3 and Equation
4 .[59] The magnitude of the streaming
potential depends on several factors, including the properties of the
liquid, the surface characteristics of the micro/nanochannel, and the
flow rate of the liquid as seen Equation 3. Here
micro/nanochannels of the NS that transport water are considered as a
capillary tube. Water absorption within porous materials is contingent
upon capillary forces, which can accelerate or deter water intrusion
into the pores.
\(V_{s}=\frac{Ɛ_{0}Ɛ_{r}\mathrm{\Delta}P\zeta}{\text{µσ}}\) Equation 1
\(I_{s}=\frac{AƐ_{0}Ɛ_{r}\mathrm{\Delta}P\zeta}{\text{µL}}\) Equation
2
\(V_{s}=\frac{4\varepsilon_{r}\epsilon_{0}\text{γζcosθ}}{\text{σµd}}\)Equation 3
\(I_{s}=\frac{\text{πγd}\varepsilon_{r}\epsilon_{0}\text{γζcosθ}}{\text{µL}}\)Equation 4
\(R=\frac{8L\mu\ }{\pi r^{4}}\) Equation 5
where L is capillary tube length, A is the cross-sectional
area, ζ is the inner surface zeta potential, ΔP external
pressure difference, µ is dynamic viscosity, Q is the
volumetric flow rate, r is the channel radius,ε0, εr, σ, γ , and μ are
the dielectric constant, conductivity, specific tension, and viscosity
of the DI water; R is the resistance of the water flow.